1. Field of the Invention
The present invention relates to an active matrix type organic EL drive circuit and an organic EL display device using the same drive circuit and, in particular, the present invention relates to an active matrix type organic EL display device of a portable telephone set or a PHS, etc., which is capable of reducing a mounting area of drivers for an active matrix type organic EL display panel and improving luminance of organic EL elements and which is suitable for a high luminance color display.
2. Description of the Prior Art
It has been known that an organic EL display device, which realizes a high luminance display by spontaneous light emission, is suitable for a display on a small display screen and the organic EL display device has been attracting public attention as the next generation display device to be mounted on a portable telephone set, a PHS, a DVD player or a PDA (Personal Digital Assistants), etc. Known problems of such organic EL display device are that luminance variation thereof becomes substantial when it is driven by voltage as in a liquid crystal display device and that, since there is difference in sensitivity of organic EL element between R (red), G (green) and B (blue) display colors, a control of luminance of a color display becomes difficult.
In view of these problems, an organic EL display device using current drive circuits has been proposed recently. For example, JPH10-112391A discloses a technique in which the luminance variation problem is solved by employing a current drive system.
An organic EL display panel of an organic EL display device for a portable telephone set, a PHS, etc., having 396 (=132×3) terminal pins for column lines and 162 terminal pins for row lines has been proposed. However, there is a tendency that the number of column lines as well as row lines is further increased.
An output stage of a current drive circuit of such organic EL display panel of either the active matrix type or the passive matrix type includes a current source drive circuit, for example, an output circuit constructed with a current mirror circuit.
In the active matrix type organic EL display panel, a pixel circuit composed of a capacitor and a transistor is provided for each display cell (pixel). An organic EL element (referred to as “OEL element”, hereinafter) is current-driven by the transistor of the pixel circuit, which is driven correspondingly to a voltage stored in the capacitor thereof. The drive system is either a digital drive system in which the drive current of the OEL element is binary-controlled or an analog control in which the drive current is controlled by an analog input data. In the case of the digital drive system, a display area of a pixel is controlled by providing a sub pixel in the pixel or the color tone of the display element is controlled by dividing a light emitting time. In the case of the analog drive system, there are a voltage setting type (voltage program system) and a current setting type (current program system). In the voltage setting type system, a terminal voltage of the capacitor of each pixel circuit is set according to a voltage signal and, in the current setting type system, the terminal voltage of the capacitor is set according to a current signal.
Incidentally, in order to restrict luminance variation, a current drive circuit of a passive matrix type organic EL display panel uses current having a peak with which the OEL element having a capacitive load characteristics is initially charged to emit light earlier. On the other hand, in the active matrix type organic EL display panel, a voltage corresponding to a drive current is temporarily written in the capacitor of the pixel circuit and then the drive current corresponding to the written voltage of the capacitor is generated. Therefore, the OEL element of the active matrix type organic EL panel is not driven by a peak current. The OEL element is driven in a sequence of the write of the drive current value and then the light is emitted. That is, the write period is necessary before the light emission period. As a result, there are problems that the earlier light emission as that in the passive matrix type organic EL panel is impossible and the light emission period becomes shorter than that in the passive matrix type organic EL panel.
The writing of the drive current is usually performed by charging the capacitor of the pixel circuit, which usually has a capacitance of several hundreds pF, with a current of about 0.1 μA to 10 μA and the time required to write the drive current becomes as long as about 10% or more of a scan period. The light emitting time is reduced correspondingly, resulting in reduction of display luminance. Particularly, when the number of display pixels is increased as in VGA, SVGA or XGA, etc., in which the time control must be performed within a limited time, the previously mentioned defect becomes serious.
Further, in a case where, in order to perform a high luminance color display with pixel density as high as VGA, SVGA or XGA, etc., OEL elements of an active matrix type organic EL display panel are current-driven by drive circuits provided externally of the organic EL display panel, an area of the display panel in which the drive circuits are mounted becomes large since the number of drive circuits for R, G and B display colors, which are provided in a peripheral portion of the organic EL display panel, is very large.
In order to reduce the mounting area of the drivers in such as a liquid crystal display device, one drive circuit is provided for R, G and B pixels and pixel circuits are driven in time-division. This is realized by selecting the R, G and B pixels in a sequence and voltage-driving the sequentially selected pixels through an analog switch (transmission gate). When such analog switch system is applied to the current-drive of the active matrix type OEL elements, the voltage drop in the analog switch becomes large, so that it is impossible to generate precise drive current.